Mechanosensory Processing in the Stick Insect Antenna

Research Areas: 

Using their antennae as actively movable mechanosensory probes to sample the adjacent space, stick insects manage to navigate safely in a complex environment. We study what kind of mechanosensory information from the antennae is mediated, how it is processed, and the neural pre-motor networks that control goal-directed limb movements. Our results indicate that diverse, detailed information on antennal movement reaches thoracic networks with short delay, where both timing and rate of spike trains contribute to encoding of tactile percepts.


Methods and Research Questions: 

Our research asks how sensory information is processed by the nervous system and how it is used to shape context dependent behavior. We use the stick insect’s antennal mechanosensory system as a model to investigate this problem. The results will be transferred to robotic tactile sensor systems.

Stick insects manage to navigate dexterously and safely in complex environments, in particular the canopy of bushes. Such environments are characterized by a large range of gaps and obstacles. For near-range orientation, stick insects use their antennae as actively movable, mechanosensory probes to search and sample the adjacent space. In the task of tactually guided locomotion, they achieve much better performance than manmade robotic systems. Since both the antenna and the nervous system of the stick insect are well described and accessible for a range of electrophysiological techniques, the stick insect is an ideal system to study the coding of mechanosensory information and how it is used for the control of goal-directed behaviours, such as reaching movements. We record intracellulary from descending interneurons (DINs) that mediate information from the antenna to the prothoracic ganglion. In the thorax, these neurons connect to pre-motor networks, where their activity modulates the control of muscle activity during ongoing walking and climbing behavior. The stick insect antenna carries a large range of different mechanoreceptors, of both proprio- and exteroceptive nature. As all of these mechanoreceptors potentially contribute to encoding of location and properties of tactual stimuli, the project investigates their relative significance in the control of leg movements.

In order to characterize the properties of DINs, we use sharp microelectrodes to record intracellulary from the DIN’s axon in the connective (nerve) just before the prothoracic ganglion, i.e. near the terminals in the prothorax. At this stage, the mechanosensory information from the antenna has already passed the brain. Simultaneously, we record the summed activity of both neck connectives upstream from the intracellular recording site, in order to tell ascending from descending neural activity. These extracellular recordings allow an estimation of the summed activity of the overall, stimulus-related neural activity, but also the characterization of some individual units. By controlled deflection of an antennal joint, specific subsets of antennal mechanoreceptors can be stimulated selectively. Individual DINs are stained using fluorescent dye to obtain an anatomical description that allows matching individually identified types of neurons. At a later stage of the project, the simultaneous activity of larger sets of DINs will be recorded with multi-electrode-arrays. The results will be transferred to Artificial Neural Network models of sensorimotor processing in goal-directed behaviour, but also to bio-inspired robotic tactile sensors.


Intracellular recordings were achieved from a set of different mechanosensory DINs responding to movements of the scape pedicel joint. Group 1 DINs were modulated in their activity depending on antennal position. Group 2 DINs were inhibited during change of antennal position. All of these inhibited DINs showed a stronger inhibition during faster movements, and were thus velocity-sensitive. Group 3 DINs showed different excitatory responses to changes in antennal position. Some of these were directionally sensitive, others position sensitive, and yet others showed a combined sensitivity. Several group 3 DINs were sensitive to changes in antennal movement velocity. Most DINs responded to antennal movement with a delay shorter than 10ms, indicating a direct pathway from the deutocerebrum (where the sensory afferents terminate) to the prothorax, with very few synapses between the sense organs in the antenna and the motorneurons that drive the muscle activity. The diversity of information reaching thoracic networks suggests that antennal mechanosensory information is further processed in the prothoracic ganglion, maybe at the level of motor networks.